This invention relates to development systems for electrostatic processors and, more particularly, to development systems for providing a magnetic field free development zone with an anti-gravitational flow of a multi-component developer.
In a conventional electrostatic printing process of the type described in Carlson's U.S. Pat. No. 2,297,691 on "Electrophotography", a uniformly charged photoreceptor is selectively discharged in an image configuration to provide a latent electrostatic image which is then developed through the use of a finely divided, electroscopic marking material, called "toner". As is known, that process has enjoyed outstanding commercial success, especially in plain paper copiers and duplicators. Nevertheless, substantial effort and expense are still being devoted to the perfection of the process, including the development step.
The modern practice is to carry out the development step on the fly -- viz., as the photoreceptor moves through a development zone. There are various techniques for accomplishing that, but experience has shown that development systems which employ a multi-component developer are generally superior. As a matter of definition, of course, a "multi-component developer" contains relatively large or coarse "carrier" particles in addition to the aforementioned toner particles. Characteristically, the materials for the toner and carrier (or, sometimes, carrier coating) components of such a developer are selected from different positions in the triboelectric series so that advantage can be taken of a triboelectric charging process to induce electrical charges of opposite polarities on the toner and carrier particles. Moreover, triboelectric ranking is taken into account during the selection of those materials since the objective is to have the polarity of the charge triboelectrically imparted to the toner particles oppose the polarity of the charge of the latent images which are to be developed. Consequently, in operation, there are competing electrostatic forces acting on the toner particles. Specifically, the toner particles in any given unit of developer are at least initially attracted to the carrier particles, but are subject to being electrostatically stripped therefrom whenever that unit of developer is brought into the immediate proximity of or actual contact with an image bearing photoreceptor.
There are some situations which call for an upward or anti-gravitational flow of developer through the development zone. Others have previously recognized that so-called magnetic brush development systems may be used to accomplish that, but those systems necessarily have a substantial magnetic field in the development zone. Indeed, it is that field which causes the developer flowing through the development zone to collect in bristle-like stacks or, in other words, to form one or more magnetic brushes. As will be appreciated, magnetic brush development systems have recently gained widespread favor, principally because they can provide excellent solid area coverage at acceptably low background development levels. Nevertheless, those systems still require a very delicate balance to achieve that while avoiding excessive wear of the developer, streaking of the developed image, or unacceptably high levels of magnetic interference with nearby parts of the processor, all as described in more detail in U.S. Pat. No. 3,906,121 to Fraser et al., issued Sept. 16, 1975. That application is specially relevant to the subject matter under consideration here and it is, therefore, hereby incorporated by reference.